1. Field of the Invention
The present invention relates to gas turbine engines, and particularly to a compressor bleed valve for improving the control of surge in such engines.
2. Description of the Prior Art
U.S. Pat. No. 3,809,490, Harner, issued May 7, 1974, describes the on-going problem of trying to avoid surge in gas turbine engines. The solution proposed over the years has been the provision of bleed valves to bleed off compressor air at different stages of the compressor. Thus, controls, mechanical or pneumatic, are provided for anticipating a surge condition by causing the valves to be opened to thereby bleed off air before a surge condition is to happen. Thus, in high power requirement conditions, the bleed valves are maintained closed, but during low power, the bleed valves are opened.
A pneumatic bleed valve is also described in U.S. Pat. No. 5,477,673, Blais et al., issued Dec. 26, 1995. This patent describes a bleed valve in the form of a piston extending radially through a bypass flow path, and operable to bleed compressor air into the bypass flow path when the piston type valve is open. The valve may be closed when air from a source downstream of the compressor impeller is fed to the head of the piston and such air is at a higher pressure than air from a downstream stage of the compressor. The pneumatic force to close the valve acts against a spring normally urging the valve to an open position.
As the engine speed changes from low to high, the bleed valve moves from an open to a closed position gradually. If during this transition the opening becomes too small, the engine may he in a surge condition.